This Application is a U.S. National Phase Application of PCT International Application PCT/JP00/03083.
1. Technical Field
The present invention relates to an electroacoustic transducer for use in a portable communication device, e.g., a cellular phone or a pager, for reproducing an alarm sound, a melody, or an audio sound voice, responsive to an incoming call.
2. Background Art
FIGS. 18A and 18B show a plan view and a cross-sectional view, respectively, of a conventional electroacoustic transducer 200 of an electromagnetic type (hereinafter referred to as an xe2x80x9celectromagnetic transducerxe2x80x9d). The conventional electromagnetic transducer 200 includes a cylindrical housing 107 and a disk-shaped yoke 106 disposed so as to cover the bottom face of the housing 107. A center pole 103, which may form an integral part of the yoke 106, is provided in a central portion of the yoke 106. A coil 104 is wound around the center pole 103. Spaced from the outer periphery of the coil 104 is provided an annular magnet 105, with an appropriate interspace maintained between the coil 104 and the inner periphery of the annular magnet 105 around the entire circumference thereof. The outer peripheral surface of the magnet 105 is abutted to the inner peripheral surface of the housing 107. An upper end of the housing 107 supports a first diaphragm 100 which is made of a non-magnetic disk so that an appropriate interspace exists between the first diaphragm 100 and the magnet 105, the coil 104, and the center pole 103. In a central portion of the first diaphragm 100, a second diaphragm 101 which is made of a magnetic disk is provided so as to be concentric with the first diaphragm 100.
Now, the operation and effects of the above-described conventional electromagnetic transducer 200 will be described. In an initial state where no current flows through the coil 104, a magnetic path is formed by the magnet 105, the second diaphragm 101, the center pole 103, and the yoke 106. As a result, the second diaphragm 101 is attracted toward the magnet 105 and the center pole 103, up to a point of equilibrium with the elastic force of the first diaphragm 100. If an alternating current flows through the coil 104 in this state, an alternating magnetic field is generated in the aforementioned magnetic path, so that a driving force is generated on the second diaphragm 101. Such driving force generated on the second diaphragm 101 causes the second diaphragm 101 to vibrate from its initial state, along with the fixed first diaphragm 100, due to an interaction with a attraction force which is generated by the magnet 105. This vibration transmits a sound.
A resonance frequency of the electromagnetic transducer 200 having the above-described structure depends on the deformation of the first diaphragm 100 in a state where the elastic force of the first diaphragm 100 and the attraction force which is generated on the second diaphragm 101 by the magnet 105 are at equilibrium.
FIG. 19 illustrates the relationship between a force-displacement curve of the first diaphragm 100 and the attraction force generated on the second diaphragm 101 by the magnet 105. The vertical axis of the graph represents the force, whereas the horizontal axis of the graph represents the displacement of the first diaphragm 100. As shown in FIG. 19, the force-displacement curve of the first diaphragm 100 and the attraction force curve (generated by the magnet 105 on the second diaphragm 101) intersect each other at an intersection A. In other words, the intersection A shows a point at which the elastic force and the static attraction are at equilibrium. The resonance frequency is determined by the elastic constant of the first diaphragm 100 at the intersection A.
In order to decrease the resonance frequency, it is necessary to increase the mass of the vibrating system (i.e., the first diaphragm 100 and the second diaphragm 101) or decrease the elastic constant of the vibrating system. However, it is undesirable to increase the mass of the vibrating system because it results in a decrease in the efficiency of the electromagnetic transducer 200. On the other hand, decreasing the elastic constant of the vibrating system too far would produce a force-displacement characteristic curve shown by the broken line in FIG. 19, which does not intersect the attraction force curve (generated on the second diaphragm 101 by the magnet 105). As a result, the second diaphragm 101 will be attracted, along with the first diaphragm 100, onto a magnetic circuit without establishing equilibrium at any position.
In other words, since the elastic constant must be kept within a range such that the elastic constant curve intersects the attraction force curve, there is a lower design limit to the resonance frequency. Although it becomes possible to decrease the elastic constant by decreasing the attraction force as well, this results in a decrease in the generated driving force, so that a sufficient reproduced sound pressure level cannot be obtained.
An electromagnetic transducer according to the present invention includes: a first diaphragm disposed so as to be capable of vibration; a second diaphragm disposed in a central portion of the first diaphragm, the second diaphragm being made of a magnetic material; a yoke disposed so as to oppose the first diaphragm; a center pole disposed between the yoke and the first diaphragm; a coil disposed so as to surround the center pole; a first magnet disposed so as to surround the coil; and a second magnet disposed on an opposite side of the first diaphragm from the center pole.
In one embodiment of the invention, the electromagnetic transducer further includes: a first housing for supporting the first diaphragm; and a second housing in which the second magnet is disposed.
In another embodiment of the invention, the second magnet has a disk shape.
In still another embodiment of the invention, the second magnet has an annular shape.
In still another embodiment of the invention, an outer diameter of the second magnet is equal to or smaller than an outer diameter of the second diaphragm in the case of the second magnet having a disk shape.
In still another embodiment of the invention, an outer diameter of the second magnet is equal to or greater than an outer diameter of the second diaphragm in the case of the second magnet having an annular shape.
In still another embodiment of the invention, the electromagnetic transducer further includes a third magnet in a central portion of at least one face of the first diaphragm or the second diaphragm.
In still another embodiment of the invention, the second magnet is magnetized in the same direction as the first magnet.
In still another embodiment of the invention, the second magnet is magnetized along a radial direction with respect to an axis through a center of the center pole.
In still another embodiment of the invention, the second diaphragm has a thickness which allows a magnetic saturation to occur when the second diaphragm is deflected toward the center pole by a predetermined distance.
In still another embodiment of the invention, the first diaphragm is made of a magnetic material.
In still another embodiment of the invention, the first diaphragm is made of a non-magnetic material.
In still another embodiment of the invention, the electromagnetic transducer further includes a first magnetic plate provided between the first magnet and the first diaphragm.
In still another embodiment of the invention, the first magnetic plate has an annular shape.
In still another embodiment of the invention, the electromagnetic transducer further includes a second magnetic plate disposed on the second magnet.
In still another embodiment of the invention, the second magnetic plate has a disk shape.
In still another embodiment of the invention, the second magnetic plate has an annular shape.
In still another embodiment of the invention, the first diaphragm is shaped so as to provide non-linear displacement characteristics for canceling non-linearity of a driving force generated on the second diaphragm.
In still another embodiment of the invention, there is a substantially linear relationship between a resultant of a first attraction force and a second attraction force and a distance between the second diaphragm and the center pole; wherein the first attraction force is a attraction force generated on the second diaphragm by a magnetic circuit including the first magnet, the center pole, and the yoke, and the second attraction force is a attraction force generated on the second diaphragm by the second magnet.
In still another embodiment of the invention, the first diaphragm is affixed by being adhered to the first housing.
In still another embodiment of the invention, the first diaphragm is affixed by being interposed between the first housing and the second housing.
In still another embodiment of the invention, the second housing is a cover for protecting the first diaphragm and the second diaphragm.
In another aspect of the invention, there is provided a portable communication device including any one of the aforementioned electromagnetic transducers.
In one embodiment of the invention, the portable communication device further includes a third housing having a sound hole therein, wherein the electromagnetic transducer is disposed so that the first diaphragm and the second diaphragm oppose the sound hole.
In another embodiment of the invention, the second magnet is disposed in the third housing.
Thus, the invention described herein makes possible the advantage of providing an electromagnetic transducer which is capable of reproducing low-frequency ranges without necessitating a change in the size of the first magnet, or the first and second diaphragms, and which is capable of reproducing a sound at a high level and low distortion by virtue of an increased driving force.